IPA Command objects sometimes need to pass some data between their
various methods. Currently that's done using the thread-local context.
For an example see dnsrecord_del, which sets a "del_all" flag in
pre_callback and then checks it in execute.

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While that works for now, it's far from best practice. For example, if
some Command can call another Command, we need to carefully check that
the global data isn't overwritten.

The other way data is passed around is arguments. The callback methods
take a bunch of arguments that are the same for a particular Command
invocation (ldap, entry_attrs, *keys, **options). By now, there's no
hope of adding a new one, since all the callbacks would need to be
rewritten. (Well, one could add an artificial option, but that's clearly
not a good solution.)
In OOP, this problem is usually solved by putting the data in an object
and passing that around. Or better, putting it in the object the methods
are called on.

This got me thinking -- why do we not make a new Command instance for
every command invocation? Currently Command objects are only created
once and added to the api, so they can't be used to store per-command data.
It seems that having `api.Command.user_add` create a new instance would
work better for us. (Of course it's not the best syntax for creating a
new object, but having to change all the calling code would be too
disruptive).